1. Field of the Invention
The present invention relates to an image inspection apparatus, an image forming apparatus, and image inspection method.
2. Description of the Related Art
In recent years, inspection apparatuses configured to perform inspection of printed matters, which has conventionally been performed manually, have come into use. Such an inspection apparatus typically determines whether a printed matter, which is an inspection subject, has a defect by the following method. A master image to be used as a reference image is generated by scanning an image of a desired quality manually selected from images of printed matters. A portion of the printed matter is compared against a corresponding portion of the master image to determine a magnitude of difference therebetween. Whether the printed matter has a defect is determined based on the magnitude of the difference.
However, plateless printers such as electrophotographic printers which are coming into widespread use in recent years are mainly used to print a small number of copies or in variable printing, in which contents to be printed vary from one page to another. Accordingly, it is inefficient to generate master images from printed matters produced by a plateless printer, unlike cases of offset printers. To solve this problem, a scheme of generating a master image from a print target image (print data) can be employed. This scheme allows efficient inspection of printed matters produced by variable printing.
When an image is formed on a white sheet of paper (hereinafter, “sheet”), inspection can be performed by making comparison between such a master image as that described above and a scanned image obtained by scanning the image formed on the sheet. Meanwhile, in a case where a printing way (hereinafter, sometimes referred to as “preprint printing”) of printing a print target image on a preprinted sheet where a predetermined preprint image such as a line frame or a template document is printed is adopted, appropriate inspection cannot be achieved only by such simple comparison as that described above. This is because, in preprint printing, while the scanned image contains the preprint image and the print target image, a master image generated from the print target image contains only the print target image but does not contain the preprint image.
An example of an image inspection method for such preprint printing is disclosed in Japanese Laid-open Patent Publication No. 3-281276. According to this method, a master image containing a preprint image and a print target image is generated by combining the preprint image and the print target image, and comparison is made using the master image. Another example method is disclosed in Japanese Laid-open Patent Publication No. 11-78183. According to this method, a masked scanned image containing only a print target image is generated by masking a portion corresponding to a preprint image on a scanned image, and comparison is made against a master image containing only the print target image. Still another example method is disclosed in Japanese Laid-open Patent Publication No. 2005-41122. According to this method, a print target image is inspected by obtaining difference between a scanned image and a preprint image and obtaining difference between the scanned image and the print target image.
When an image is formed by an image forming apparatus configured to form an image on a sheet being conveyed, the image can be out of registration with respect to the sheet or, in other words, what is referred to as “registration shift” can occur. Generally, magnitude of this registration shift is as small as several pixels, which is substantially visually unrecognizable. However, the registration shift of several pixels can affect considerably on a result of the above-described image inspection that is performed based on comparison between corresponding pixels.
Accordingly, there is a need for performing registration between a master image and a scanned image when image inspection is performed by comparing corresponding pixels between the master image and the scanned image. In a case where an image is formed/output on a white sheet, registration can be performed between an entire scanned image and an entire master image.
However, if registration shift should occur in preprint printing, a print target image goes out of registration with respect to a preprint image. In other words, if registration shift should occur, because a master image becomes different from a scanned image in terms of image, the master image will not be coincided with the scanned image only by adjusting positions of the entire images. This makes registration difficult.
The technique disclosed in Japanese Laid-open Patent Publication No. 11-78183 will not pose a problem so long as the preprint image can be masked accurately. However, if a part of the preprint image remains unmasked due to inaccurate masking, this part is erroneously detected as a defect. Furthermore, in a case where the preprint image and the print target image are in considerably close proximity to each other or in a case where the images overlap, the print target image can be partially undesirably masked, resulting in erroneous detection of a defect.
The technique disclosed in Japanese Laid-open Patent Publication No. 2005-41122 has not only a disadvantage similar to the problem of the technique disclosed in Japanese Laid-open Patent Publication No. 11-78183 but also a disadvantage in terms of processing time and apparatus cost. More specifically, because the technique involves multiple difference calculations, processing time is increased due to the calculations. Furthermore, because this technique requires that both a master image for the preprint image and a master image for the print target image be stored and that differential data of multiple versions be stored, the need of increasing an on-board memory in size will arise. The multiple difference calculations may be performed in parallel to reduce processing time. However, in this case, it is necessary to add processing logic therefor.
Therefore, there is a need for an image inspection apparatus, an image forming apparatus, and an image inspection method that are capable of inspecting a printed matter produced by forming an image additionally on a sheet, on which another image has already been formed, accurately with simple structure.